Timelion error: Cannot run exists query on _field_names

I recently discovered that one of our Timelion visualizations is no longer working.

Timelion: Error: in cell #1: [unsupported_operation_exception] unsupported_operation_exception: Cannot run exists query on _field_names

Here's the Timelion code:

.es(index=jira)

The error only occurs on this particular index (as far as I can tell) and _field_names are not disabled for this index.

Not sure when it stopped working, but maybe when we upgraded from 6.7 to 7.0.

Elasticsearch and Kibana all at 7.3.0.

Any ideas on what can be wrong here?

Hmm, do you have any exists filters or queries?

No, this is from simply creating a new Timelion visualization and entering .es(index=jira) in the form and pressing "play".